bpo-30040: update news entry (GH-12324)

methane · web-flow · commit 3fe7fa316f74 · 2019-03-14T18:54:09.000+09:00
This optimization is not only for space, but also for speed.
diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-03-11-22-30-56.bpo-30040.W9z8X7.rst b/Misc/NEWS.d/next/Core and Builtins/2019-03-11-22-30-56.bpo-30040.W9z8X7.rst
@@ -1,2 +1,3 @@
 New empty dict uses fewer memory for now.  It used more memory than empty
-dict created by ``dict.clear()``.  Patch by Inada Naoki.
+dict created by ``dict.clear()``.  And empty dict creation and deletion
+is about 2x faster.  Patch by Inada Naoki.
